As known, there is apparatus which, for its operation, requires a determination of the displacement of movable bodies with respect to fixed reference elements situated in zones distant from the apparatus and in environments where the atmospheric conditions are severe and changeable. For example, the central control unit of an aircraft, situated in the pilot compartment, must know the position of the ailerons, and it is evident that the environmental conditions around the aircraft, such as temperature, pressure, humidity and so on, may be extremely variable.
It is also known that the most reliable instruments to measure such displacements are those of the optical type which, besides being simple and compact, have a low attenuation of the signal and have an immunity to electromagnetic disturbances.
Further, optical position sensors are known, which make use of a local, amplitude modulated monochromatic light source which, through an optical fiber, is conveyed to the remote area where the movable body whose position is to be determined is located. In said sensors, the modulated light exits from the free end of the optical fiber, strikes the surface of the movable body and is reflected into the optical fiber.
In this way, the light reflected by the movable body is phase displaced by an angle which is proportional to the distance between the movable body and the end of the optical fiber. The phase displaced light transits the optical fiber again and is locally compared to the modulating signal. In this way, the phase displacement of the reflected light with respect to the modulating signal, and therefore, the position of the movable body, is determined.
The above-described optical sensors have a substantial drawback in that the phase displacement measured between the reflected light and the modulating signal does not represent only the position of the remote movable body, but also involves all of the undesirable phase variations induced by the environmental conditions where the optical fiber is operating.
In fact, such variations in temperature, pressure, humidity and so on, together with the length of the optical fiber, which can be some tens of meters, induce variations in the transmission characteristics of the optical fiber, which result in phase displacement of the optical signal not due solely to the position of the movable body. In particular, when the displacements to be measured are very small, of the order of some tens of microns, the false or environmental phase displacements induced by the ambient can distort the measurement of the position of the remote body.